There are two main types of tectonic plates: Oceanic and Continental. Continental crust is mostly formed of granite and is commonly referred to as sial due to a large amount of silica and aluminium present. Continental crust is less dense than the basaltic oceanic crust, sima, which is comprised of silica and magnesium. Because it is denser, oceanic crust plunges beneath the continental crust. The division between the two layers is known as the Conrad Discontinuity.
